1. Field in the Industry
The present invention concerns a weldable steel of high strength and high toughness. The invention also concerns a method of producing steel members for the parts such as automobile parts using the steel.
2. State of the Art
In the event of manufacturing various machine parts with steel materials, if two or more components can be welded to form the shape of the part, it will be easy to manufacture complicated products. Then, it is possible to reduce the number of the parts by gathering two or more components, which have been to date combined with bolts and nuts, into one part, and resulting decrease in weight of the parts and reduction of the manufacturing costs will be enjoyable. However, in cases where high strength and high toughness are required to the parts, there is a problem that the steel of such properties has poor weldability and thus, it is difficult to manufacture desired parts by combining particular members. One falls into dilemma that, for the purpose of heightening weldability of the steel it is necessary to choose an alloy composition of lower carbon content, while the low carbon steel has lower hardness, lower toughness and lower strength.
In order to maintain the weldability of the steel high it is essential not to lower the toughness of the heat-effected parts around the welded parts. In general, hardness of the heat-effected parts goes to too high a level of 400 HV due to martensitic transformation caused by heat given during welding and rapid cooling thereafter, and then, the heat-effected parts become brittle and weld-cracking may occur. Because the hardness of the steel after the martensitic transformation depends mainly on the carbon content, it is necessary to keep content of the components, particularly carbon, which increase hardness low, for the purpose of avoiding occurrence of extremely hard heat-effected parts. From this point of view an index for keeping the contents of the components which increase the hardness low “Index of Weld-Cracking Susceptibility” (hereinafter abbreviated to “Pcm”) is known and used.
On the other hand, too low a carbon content makes the strength of the steel insufficient. A countermeasure for this problem is to increase hardenability of the steel by, while maintaining the C-content, regulating the contents of the other alloying elements so that the depth of hardened layer may be deep and the averaged hardness of the welded products may be high, thereby to maintain the strength of the products. From this point of view, an index for deciding the minimum contents of the alloying elements which influence the hardenability, “Manganese Equivalent” has been discussed (hereinafter abbreviated to “Mneq”).
In regard to the steel of low yield ratio-high strength used for architectural constructions or large scale structure such as bridges there has been proposed a steel having a specific alloy composition, a structure consisting of (by volume %) polygonal ferrite 5-30%, MA (mixture of martensite and austenite) 3-15% and the balance of bainite, and the averaged size of the MA is up to 5 micrometer, as a material having good toughness and the weldability (Japanese patent disclosure No. 2004-315925). The patent literature, however, discloses in regard to the weldability only the results of heat-cycle tests which simulate welding (HAZ-toughness).
The inventors made research to seek the way of keeping the toughness at the heat effected parts at manufacturing steel parts in which the base metal maintains the necessary strength and toughness with the requisites that the above noted two indices related to the weld-crack susceptibility and the hardenability are chosen to be appropriate values. They discovered a useful steel of specific alloy composition and found that application of specific processing conditions to the steel makes it possible to solve the above noted problem.